bug#56546: 29.0.50; unbounded RAM comsumption when displaying images

[Eli Zaretskii]

> From: "Jose A. Ortega Ruiz" <mail@jao.io>
> Date: Thu, 14 Jul 2022 02:35:46 +0100
> 
> 
> It seems to be easy to make emacs (under X) to consume more and more
> RAM, which is never released, by making it display images.  A extreme
> (in my experience) case is animated GIFs, try:
> 
>   - emacs -Q
>   - M-x eww RET https://xenodium.com/emacs-zones-to-lift-you-up/
>   - RAM consumption grows to ~600Mb
>   - R (redisplay page): RAM grows to ~1100Mb
>   - R (redisplay page): RAM grows to ~1752Mb
>   - R (redisplay page): RAM grows to ~2222Mb
>   - rinse and repeat: RAM never goes down
>   - (image-cache-size) reports a modest 82Mb
>   - Kill buffer:  high RAM consumption is still at its maximum, even
>     after (image-cache-size) goes to 0

Run some utility that displays the memory-map of an application, and
you will see that most of that memory is free for use.  Emacs just
didn't release it to the OS, but kept it in the memory pages allocated
to the process, for future allocations.

The strategy for releasing memory to the OS is in glibc, not under our
control.  Last time we talked with glibc developers, they maintained
that this is the expected and correct behavior.